Acoustic generator of the spark discharge type



Sept. 24, 968 H A WRlGHT JR ET AL 3,403,375

ACOUSTIC GENERATGR OF THE SPARK DISCHARGE TYPE Filed April 27, 1957 lil@- 1 N VENTORS nted States Patent O 3,403,375 ACOUSTIC GENERATOR OF THE SPARK DISCHARGE TYPE Hubert A. Wright, Jr., Lexington, Mass., and John P.

Tobey, Jr., Nashua, N.H., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Apr. 27, 1967, Ser. No. 637,033 4 Claims. (Cl. 340-12) ABSTRACT OF THE DISCLOSURE The present invention is ldirected to an impulse acoustic generator of the type which utilizes as the sound generating mechanism a controlled electrical discharge that takes place between two volumes of an electrically conductive fluid which are separated by a dielectric barrier having an aperture of small cross section formed therein.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

Summary of invention Impulse sound sources have been used with good results in geophysical exploration and in sonar systems because of their ability to yield improved target classification and position resolution. In this type of generator, the sound pulse may be generated by high explosives, underwater electrical sparks or electromechanical and mechanical devices. Each of these techniques has associated disadvantages. For example, high explosives present a handling and safety problem, mechanical devices are deteriorated by cavitational pitting and electrical spark `devices suffer from erosion of the electrodes during discharge. The latter phenomenon limits the life of the apparatus and additionally degrades its ability to reproduce the output pulse.

The present invention solves the problem of electrode deterioration in spark devices by employing an arrangement wherein the spark is struck and maintained at a point remote from the electrodes. More particularly, the acoustic generator herein disclosed, which may be considered an electrodeless spark device, initiates the spark between two volumes of an electrolyte which are separated by a dielectric barrier having a small aperture formed therein. This aperture, in one preferred embodiment, has a discontinuous geometry which is bell-shaped. This bellshaped configuration, it has been found, increases the amplitude of the -output pulse that can be obtained with a given level of excitation voltage.

Although metallic circuit electrodes must be used in the apparatus to conduct current from the external circuit into the electrically conducting fluid, these electrodes are of large area so that the current density at their surfaces is low and sparking is prevented from taking place there. The substitution lof electrolyte for the metal electrodes in the arc zone allows the replacement of the eroded fluid after each acoustic pulse. Thus, while some erosion does exist in the electrodes, each electrode is replenished constantly by a readily available fluid supply.

It is accordingly a primary object of the present invention to provide an impulse acoustic generator which employs a spark discharge but `which does not suffer from electrode deterioration as a consequence of repeated spark discharges.

Another object of the present invention is to provide an impulse sound s-ource which utilizes an electric discharge that takes place between two volumes of an elec- 3,403,375 Patented Sept. 24, 1968 ice trolyte which are in communication only via an aperture of small cross section.

Another object of the present invention is to provide an acoustic sound source of the spark discharge type wherein the spark discharge occurs remote from the electrodes.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 illustrates the principle of operation of the impulse sound gener-ating apparatus of the present invention; and

FIG. 2 shows a preferred embodiment of the generator wherein the communicating aperture between the two volumes of the electrically conducting uid has a bell-shaped configuration.

The principle of operation of the present invention and the manner in which it differs from conventional spark discharge generators may, perhaps, best be understood by referring now to FIG. 1 of the drawings. It Awill be seen that the overall system includes an enclosed container 1 made of electrically nonconductng material and fabricated with an intermediate barrier 2 having a central restricted aperture 3 formed therein. In operation, the complete interior of container 1 is filled with an electrolyte 4 which may, for example, be seawater. Positioned on lopposite sides of aperture 3 and supported from the end walls of container 1 are a pair of aligned electrodes 5 and 6. The area of these electrodes is large compared to the cross section of aperture 3. These electrodes, as is well known in the spark generator art, may be energized by means of a suitable control switch 7 which, in the position shown, connects these electrodes across a capacitor bank 8 and, in an alternative upper position, permits this capacitor bank to be charged from a suitable DC power supply 9.

The mode of operation of the arrangement of FIG. l is as follows: First, switch 7 is moved to its upper position as seen in this ligure and capacitor bank 8 charged to a voltage level which may be, for example, approximately thirty kilovolts. Once capacitor bank 8 is fully charged, switch 7 is moved to the position shown and, with the voltage `available at capacitor bank 8 acting across aperture 3, a spark breakdown occurs through this aperture. The energy stored in the capacitor bank is thereafter delivered to the spark, rapid spark growth results, and an accompanying acoustic pulse is radiated through container 1 into the surrounding fluid medium.

Since electrodes 5 and `6 are made with an area large enough to maintain their surface current density below the point of sparking, when the spark is initiated, it occurs between two Volumes of the electrolyte on opposite sides of barrier 2. This discharge which starts at a location remote from the electrodes never reaches or contacts these elements. Consequently, electrode erosion because of sparking is eliminated. Because of this, it will be appreciated, the apparatus of FIG. 1 -can repeatedly generate acoustic pulses having substantially identical wave forms. This characteristic is, of course, extremely desirable in sonar systems, for example, since it permits successive echo pulses to be compared for target classification purposes.

An alternative embodiment of the present invention is shown in FIG. 2. Here, an inner cup 20 of electrically conducting material serves as one of the electrodes and a metal faceplate 21 having a central aperture 35 formed therein serves as the other electrode of the generator.

Inner electrode 20 is accommodated within a metallic housing 22 and it is electrically isolated therefrom by an appropriate insulating member 23. 'Separating the inner and outer electrodes is a dielectric spacer 24 which functions in a manner similar to barrier 2 in FIG. 1. This spacer has a central aperture 25 formed therein which is in alignment with the aperture formed in faceplate 21.

Faceplate 21 is held in place by means -of a multiplicity of bolts, such as 26 and 27, which pass through suitable clearance holes in this plate and thread into locking means 28 and 29 secured to the outer wall surface of housing 22. To prevent the fluid which fills the interior of inner electrode when the generator is immersed in seawater from reaching housing 22 and shorting the device, sealing rings 30, 31 and 32 are used at the interface of inner cup 20, insulator 23, ,and the dielectric spacer 24.

Energization of the generator may be accomplished by an arrangement similar to the one shown in FIG. 1. To this end cable 32, which passes `through theI base of housing 22, through insulator 23 to the bottom of inner electrode 20, has its conductor 34 electrically connected to the inner electrode by any suitable means. The other conductor, not4 shown, may be connected either to housing 22 or faceplate 21 since 'these elements are electrically interconnected by means of the holding bolts 26 and 27. Alternatively, the seawater path may be used as the second conductor when the apparatus is immersed.

The mode of operation' of the modification of FIG. 2 is essentially the same as that of FIG. l except that in the apparatus just described there is no fluid compartment on the top side of the dielectric spacer 24 but an unbounded fluid medium. The spark discharge, however, is still initiated between' Volumes of electrolyte which are on opposite sides of aperture 25. It would be noted in this connection that the dimensions of aperture 35 in faceplate 21 and the dimensions of inner cup 20 are again made large enough in comparison to the aperture in spacer 24 so that their surface current density is below the point of sparking or breakdown.

The geometric shape of the aperture in the dielectric spacer through which the spark discharge takes place has been found to be an extremely important factor in the operation of the overall device. The electric resistancevof the entire external circuit is determined primarily by the resistance of the electrolyte in series with the spark. The electrolyte resistance, however, is determined by the conductivity of the electrolyte and the shape of the aperture throat.

There is some e-vidence that the acoustic pressure generated by the electrodeless spark is proportional to the second time derivative of the internal energy of the spark. Thus, an aperture geometry which results in a discontinuous rate of power dissipation would appear to yield the best results. The apparatus of FIG. 2, therefore, employs a bell-shaped geometry, that is, aperture may be looked upon as having an upper, bell-shaped portion 36 and a confronting lower, bell-shaped portion 37. Each bell-shaped portion has a flared skirt, such as 38, which narrows to a dome-shaped portion 39. Both bell-shaped portions meet at the center of the dielectric spacer to define a throat 40, the narrowest opening of the passageway between the interior of the generator and the external fluid medium. Tests of such bell-shaped apertures and other discontinuous geometries show that these configurations significantly increase .the acoustic pressure generated.

When exposed to too intense a spark discharge, the aperture dielectric plates fail either by cracking or -by gradual thermal or mechanical erosion. The aperture plates which have been used most 'successfully have been either plastic base composites or refractory ceramics. Composites such as an epoxy-fiberglass and a Teflonquartz have the advantage of great mechanical strength and relatively low cost.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: 1. An impulse sound generator comprising, in combination,

a first cup-shaped electrode; a second electrode having an aperture therethrough; a dielectric spacer having an aperture therethrough; said electrodes and said dielectric spacer being secured together with said dielectric spacer insulating said first electrode from said second electrode and with a fluid passageway extending from the interior of said cup-shaped electrode through the apertures in said dielectric spacer and said second electrode out into the surrounding medium; means for electrically insulating the exterior of said cup-shaped electrode from said surrounding medium;

the size of the aperture in said dielectric spacer, the

diameter of said cup-shaped electrode, and the diameter of the aperture in said second electrode being such that whenever the surrounding medium is an electrically conducting uid and a suitably high voltage is connected across said electrodes their surface current density is below the point of sparking and the spark discharge, when it occurs, takes place through the aperture in said dielectric spacer and between volumes of the electrically conducting fluid located on opposite sides of the aperture in this dielectric spacer.

2. In an arrangement as defined in claim 1 wherein the aperture in said dielectric spacer has a geometry which results in a discontinuous rate of power dissipation when said spark discharge occurs.

3. In an arrangement as defined in claim 1 wherein the aperture in said dielectric spacer has a bell-shaped geometry.

4. In an arrangement as defined in claim 1 wherein the aperture in said dielectric spacer has a cross-sectional shape that is similar to two bell shapes end-toend,

said bell shapes meeting at the center of said dielectric spacer and defining a throat which is the narrowest opening in the fiuid passageway, extending from the interior of said cup-shaped electrode out into the surrounding medium.

FOREIGN PATENTS 2/1960 Germany.

RODNEY D. BENNETT, Primary Examiner. B. L. RIBANDO, Assistant Examiner. 

